1. Field of the Invention
The present invention relates to a semiconductor device provided with an active circuit comprising a thin-film transistor (hereunder referred to as “TFT”) on a substrate with an insulating surface. The invention may be used with particular advantages in electro-optical devices, a typical one being a liquid crystal display device having an image display region and its driver circuit fowled on the same substrate, and in electro-optical device-mounted electronic instruments. Throughout the present specification, “semiconductor device” will refer to general devices that function based on semiconductor properties, and it will include in its scope the aforementioned electro-optical devices and electronic devices having the electro-optical devices mounted thereon.
2. Description of the Related Art
TFTs having semiconductor layers formed with crystalline silicon films (hereunder referred to as “crystalline silicon TFTs”) have high field-effect mobility, and are therefore capable of forming circuits with various functions. Active matrix-type liquid crystal display devices employing crystalline silicon TFTs have an image display region and a driver circuit for image display formed on the same substrate. In the image display region there are provided a pixel TFT formed by an n-channel TFT, and a storage capacitor, while the driver circuit is constructed with a shift register circuit, level shifter circuit, buffer circuit, sampling circuit or the like, which is formed based on a CMOS circuit.
However, the operating conditions are not the same for the pixel TFT and the driver circuit TFT, and therefore different properties are often required for the TFTs. For example, the pixel TFT functions as a switch element and is driven by application of a voltage to the liquid crystals. Because the liquid crystals are driven by alternating current, it is most common to employ what are known as frame inversion driving systems. In such systems, the pixel TFT is required to have the property of a sufficiently low off-state current value (the drain current flowing when the TFT is off) in order to minimize power consumption. On the other hand, since a high driving voltage is applied to the buffer circuit of the driver circuit, it is necessary to increase the voltage resistance to prevent breakage upon application, of the high voltage. Increased current driving capacity requires a sufficient guarantee for the on-state current value (the drain current flowing when the TFT is on).
The lightly doped drain (LDD) structure is known as a structure for a TFT exhibiting a reduced off-state current value. This structure is provided with a region having an impurity element added at a low concentration between a channel-forming region and a source region or drain region formed by addition of an impurity element to a high concentration, and this region is called the “LDD region”. One means known for preventing deterioration of the on-state current value due to hot carriers is a structure known as a GOLD (Gate-drain Overlapped LDD), wherein the LDD region is placed lying over the gate electrode with a gate insulating film therebetween. This type of structure is known to be effective for preventing inclusion of hot carriers by attenuation of high voltage near the drain, thus avoiding the deterioration phenomenon.
At the same time, demands are increasing for larger sized and more intricate screens, to give greater product value to active matrix-type liquid crystal display devices. However, the larger sizes and greater intricacy of screens increases the number and length of the scanning lines (gate wirings), thus heightening the necessity for low resistance of the gate wirings. That is, as the number of scanning lines increases, the charging time for the crystals is shortened, such that the time constant for the gate wiring (resistance x capacity) must be reduced for a faster response. For example, if the resistivity of the material forming the gate wiring is 100 μΩcm the limit to the screen size will be about 6 inches, but for 3 μΩcm a display corresponding to 27 inches is possible.
Still, the properties required for a pixel TFT of a pixel matrix circuit and a TFT of a driver circuit such as a shift register circuit or buffer circuit are not always the same. For example, in a pixel TFT, a large reverse bias (a negative voltage in the case of an n-channel TFT) is applied to the gate, but a driver circuit TFT will basically fail to operate in a reverse bias state. The operating speed of a pixel TFT is also sufficient at less than 1/100 that of a driver circuit TFT.
In addition, while a GOLD structure provides a strong effect of preventing on-state current value deterioration, it has also presented the problem of a larger off-state current value compared to the usual LDD structure. Thus, it has not been a preferred structure for application to pixel TFTs. Conversely, the usual LDD structure has a strong effect of minimizing the off-state current value but has had a low effect of preventing deterioration due to hot carrier inclusion by attenuation of the electric field near the drain. Consequently, it has not always been preferable to form all the TFTs with the same structure in semiconductor devices comprising multiple integrated circuits with different operating conditions, such as active matrix-type liquid crystal display devices. These problems have become more conspicuous particularly in crystalline silicon TFTs with higher characteristics, and as greater performance has been required for active matrix-type liquid crystal display devices.
The use of aluminum (Al) and copper (Cu) as wiring materials has been considered for realization of large-sized active matrix-type liquid crystal display devices, but this has presented drawbacks such as poor corrosion resistance and heat resistance. Consequently, these materials are not necessarily preferred for formation of TFT gate electrodes, and it has not been easy to introduce such materials into the TFT manufacturing process. Wirings can of course be formed with other conductive materials, but there are no materials with such low resistance as aluminum (Al) and copper (Cu), and this has hampered fabrication of large-sized display devices.